1. Field of the Invention
The present invention relates to a vinyl acetylsalicylate-vinyl alcohol based copolymer suitable for use as medicament and to a copolymeric precursor thereof.
2. Description of the Prior Art
Acetylsalicylic acid is widely used as a medicament, called aspirin, for producing antipyretic and analgesic medicines. There have been many investigations on acetylsalicylic acid, and it has recently been reported that acetylsalicylic acid has some newly found effects such as antithrombotic effect in addition to the antipyretic and analgesic effects.
For broadening the scope of application of acetylsalicylic acid as medicament, attempts have been made to control as desired or stabilize the efficacy of the acid. For instance, conversion of acetylsalicylic acid to its sodium salt, aluminum salt, etc., incorporation of acetylsalicylic acid in tablets, capsules, etc. and the like methods have been attempted for the purpose of controlling and stabilizing the efficacy of acetylsalicylic acid. These methods, however, are not yet satisfactory for enlarging the application area of acetylsalicylic acid as medicament.
As another means for broadening the scope of application of acetylsalicylic acid as medicament, a vinyl acetylsalicylate polymer obtained by polymerizing vinyl acetylsalicylate has been proposed. The polymer upon hydrolysis yields acetylsalicylic acid. The hydrolysis is regulated to control the amount of acetylsalicylic acid formed, thereby controlling the efficacy of the acid, or the way the effects of the acid are exerted.
A further attempt has been made in which the vinyl acetylsalicylate is regulated in degree of polymerization or saponification, whereby the amount of acetylsalicylic acid groups in the polymer can be varied, in order to control the efficacy of acetylsalicylic acid.
However, the vinyl acetylsalicylate polymer is a homopolymer and its saponification reaction proceeds with difficulty. Therefore, it is difficult to achieve the intended control of saponification degree of the polymer, namely, to accomplish a desired control of the efficacy of acetylsalicylic acid.